Wireless communications systems comprising wireless networks, in which a user equipment (UE) such as a mobile handset communicates via wireless links to a network of base stations or other wireless access points connected to a telecommunications network, have undergone rapid development through a number of generations of radio access technology. The initial deployment of systems using analogue modulation has been superseded by second generation (2G) digital systems such as GSM (Global System for Mobile communications), typically using GERA (GSM Enhanced Data rates for GSM Evolution Radio Access) radio access technology, and these systems have themselves been replaced by or augmented by third generation (3G) digital systems such as UMTS (Universal Mobile Telecommunications System), using the UTRA (Universal Terrestrial Radio Access) radio access technology. Third generation standards provide for a greater throughput of data than is provided by second generation systems; this trend is continued with the proposals by the Third Generation Partnership Project (3GPP) of the Long Term Evolution (LTE) system, using E-UTRA (Evolved UTRA) radio access technology, which offers potentially greater capacity and additional features compared with the previous standards.
Note that the term “GERA” is used herein to refer to the radio access technology associated with GERAN (GERA networks) wireless networks, “UTRA” is used to refer to the radio access technology associated with UTRAN (UTRA networks) wireless networks, and similarly the term “E-UTRA” or “LTE” is used to refer to the radio access technology associated with E-UTRAN (E-UTRA networks) wireless networks.
LTE is designed primarily as a high speed packet switched network, and voice services, packet switched voice services and in particular Voice over Internet Protocol Multimedia Subsystem (VoIMS) services are envisaged, whereas previous generation systems such as UMTS support voice services that are primarily circuit switched.
As new technology is introduced, networks are typically deployed which include radio access networks that use a radio access technology according to a recent standard and also legacy radio access networks that use a legacy radio access technology. A user equipment may be typically capable of communication using two or more radio access technologies, so for example the user equipment is able operate using one radio access technology, perhaps offering high capacity, where this is available, but being able to operate using a legacy radio access technology, in those service areas of the network that do not support the other radio access technology, or that do not support preferred features.
There are a variety of handover procedures that have been developed to allow handover of a user equipment between a E-UTRA network and a UTRA/GERA network. In particular, handover procedures have been developed to allow handover when a voice call is in progress, that is to say so-called Voice Call Continuity (VCC) handover procedures. Typically a VCC procedure will be implemented under the control of a session transfer controller, which will typically comprise a Service Centralization and Continuity Application Server (SCC AS) and a Serving Call Session Control Function and/or Interrogating Call Session Control function (I/S-CSCF). The session transfer controller is typically implemented in the Internet Protocol Multimedia Subsystem (IMS).
The IMS is typically used to control packet switched services offered over the E-UTRA network; control of circuit switched services in a UTRA/GERA network typically involves a mobility controller, such as a Mobility Switching Centre (MSC). The session transfer controller typically communicates with the mobility controller during handover according to a VCC procedure.
A user equipment may be equipped with a single radio transceiver, for reasons of economy or for minimizing power consumption, so that simultaneous communication with two radio access networks is not possible. In this case the handover protocol typically uses a break-before-make radio connection during handover. Handover procedures known as Single Radio Voice Call Continuity (SR VCC) procedures may be available for use in some user equipments.